Micro-hardness testing of metals is widely used in industry since, in many cases, hardness values have been found to correlate with physical properties such as wear, fatigue and tensile strength. Micro-hardness testing in the traditional sense involves pressing a diamond indenter, usually a Knoop indenter in the United States or a Vickers indenter in Europe, into a sample which has been polished more or less flat, by means of a static load which is applied to the diamond. After removing the diamond from the surface an impression is left in the sample as a result of plastic deformation which occurs during the indentation process. Optical measurement of the width of this impression then provides data from which the "hardness" of the material can be calculated. The material, it should be noted, also deforms elastically; however, in correct commerical practice this elastic component is very small and is not measured.
Hardness has been defined as the static load applied divided by either the constant area (Vickers) or area of the indenter in contact with the projected surface (Knoop). The Vickers diamond is a tetragonal 136.degree. diamond; the Knoop diamond is an elongated tetrahedron. Therefore, for the Vickers indenter: ##EQU1## where d is the average diagonal distance across the indentation, a is 136.degree. and L is the applied load in Kg; and for the Knoop: ##EQU2## where L is applied load, l is the length of the long diagonal and ep is a constant related to projected area of the indention.
According to prior art procedures for measuring hardness, an optical microscope is used to measure the width of the indention. This technique imposes a limitation on the accuracy of the measurement as the resolution of an optical microscope is: ##EQU3## where .lambda. is the wave length of light and NA is a numerical aperture, as defined for example in Fundamentals of Optics, Jenkins and White, McGraw-Hill, 1957, pp. 306-307. For microscopes now typically in use, .DELTA.l is approximately 500 nm.
Accordingly, instruments and techniques of the prior art are inherently subject to error due to the limitations of optical measurement as well as errors in calibration standards, load overshoot and vibration.
It is therefore an object of this invention to provide a system for determining the hardness of a material such as a metal which avoids many of the deficiencies inherent in the prior art by continuously monitoring and registering the displacement of the indenter used to impress the sample. It is a further object of this invention to provide a system for continuously and automatically monitoring and correlating data relating to load applied to the indenter, time and displacement to obtain additional information on mechanical properties of the material.